Conventional offboard navigational devices request route data from a main station and are able to guide the user along this route. Requesting route data requires the communication of map data, or at least waypoints via an air interface which, generally, may produce only rather low transmission rates. In order to transfer the relevant data in an acceptable time via such a low-rate air interface, numerous proposals have already been made for compressing, thinning out or configuring route data.
German Published Patent Application No. 100 29 198 describes an n-fault-tolerant route, German Published Patent Application No. 101 05 899 describes a reduction to decision points and crossing points, and German Published Patent Application No. 101 05 897 describes a method for requesting route data. There, the route data are reduced to the essential data required for exactly the one route. In the first application, a route corridor is described, which, besides the main route, includes all possible first, second, third, (nth) alternative routes. In the second application, a method for thinning out a route is described in that exactly those data are left out which are necessary between crossings only for position finding, but not for navigation. In the third application, a method for requesting route data is described, in which boundary values which have already been determined, maximum storage volume, maximum route length are passed to a server.